Vacuum countergravity casting methods, such as described in U.S. Pat. Nos. 3,900,064; 4,340,108 and 4,606,396, have been in use in the casting of steel, aluminum and gray cast iron for which the molten metal chemistry can be readily controlled and maintained during the time that successive molds are immersed in the melt and the melt drawn upwardly into each mold by application of a suitable negative differential pressure between the mold and the melt. However, in the casting of nodular iron, a highly volatile, fugitive, nodularizing agent, such as magnesium, is required in the melt to effect desired spheroidization of the carbon therein during the casting process. In attempts to cast nodular iron parts using the vacuum countergravity processes described in the aforesaid patents, prior art workers have experienced great difficulty in maintaining the proper concentration (i.e., at least 0.02 percent by weight) of magnesium in the melt over the extended time required to cast a plurality of molds in succession from the melt. This difficulty is attributable to the rapid evaporation of magnesium from the melt following an initial treatment of the melt with a magnesium-bearing nodularizing agent. Erratic, uncontrolled loss (also known as fade) of the fugitive magnesium from the melt over time has been experienced and resulted in off-chemistry melts in so far as magnesium content is concerned and correspondingly inconsistent nodularization.
As a result of this inability to reliably control and maintain the melt chemistry (i.e., to maintain the magnesium content above the desired effective level) over the time required for casting a plurality of molds in succession, use of the countergravity casting processes described in the aforesaid patents in high volume production of nodular iron parts has been rendered impractical and/or uneconomical to date. In this regard, it is desirable to cast for at least five minutes before replacing the melt in the underlying pool. Magnesium fade over this period can be as much as 0.075% by weight unless some means can be provided to keep the magnesium concentration up during that period.
It is an object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a molten metal, such as nodular iron, from a melt wherein the content of a fugative alloyant, such as a magnesium nodularizing agent, in the melt is reliably controlled over time throughout the casting of successive molds therefrom.
It is another object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of molten metal, such as nodular iron, wherein the mold includes a treating agent comprising the fugative alloyant so disposed thereon as to contact the underlying melt and replenish alloyant therein to maintain a desired effective concentration of alloyant for casting, thereby counteracting loss (or fade) of the alloyant from of the melt over time.
It is another object of the present invention to provide a method of making a countergravity casting mold in such a manner as to incorporate a treating agent comprising a fugative alloyant on the lower mold portion for introduction into the melt when the mold is immersed therein.